1. Field of the Invention:
This invention is in the field of digital data communications, and more particularly, in the field of programmable communications controllers which buffer and control the transmission of data between a communication processor of a general purpose digital computer and a number of communication I/O ports of the controller to which ports terminal devices can be connected directly, or indirectly, by digital data communication lines.
2. Description of the Prior Art:
Microcomputer systems such as those developed by the Intel Corporation, the Intel 8080, which is described in detail in the Intel 8080 Microcomputer Systems User's Manual dated September 1975, have the capability of being programmed to control and buffer communications between a number of communication lines and a communication processor such as the Datanet 355 which is manufactured by Honeywell Information Systems Inc., the assignee of this invention.
Programmable communication controllers have many advantages derived from the fact that their microcomputer systems can be programmed. The chief advantage is that it is easier and less costly to change or modify a program than to change or modify hardware to permit, for example, terminals having different characteristics to be connected to such controllers or to add to or modify their operation. However, programmable communications controllers also have certain disadvantages, or limitations, when more than one communication line is serviced by such a controller which is normally the case. One such disadvantage is the requirement of testing before issuing memory or I/O commands to determine which I/O port, or communication line, is being serviced at that time. Such tests effectively slow down the microcomputer system and the communications controller, and thus limit the number of communication I/O ports, or communication lines, that can be serviced by a programmable communications controller all other parameters remaining the same.
Writing a separate program for each communication line to be serviced eliminates the need for testing to determine the communication line being serviced, but such a solution substantially increases the size of the addressable memory of the microcomputer system and, thus, the cost of the communications controller. This solution also increases the difficulty and cost of developing, testing and maintaining the necessary number of programs needed by the microcomputer system of a communications controller.